afs: Fix the handling of CB.InitCallBackState3 to find the server by UUID

Fix the handling of the CB.InitCallBackState3 service call to find the
record of a server that we're using by looking it up by the UUID passed as
the parameter rather than by its address (of which it might have many, and
which may change).

Fixes: c35eccb1 ("[AFS]: Implement the CB.InitCallBackState3 operation.")
Signed-off-by: David Howells <dhowells@redhat.com>